The present invention relates to ocean platforms and more particularly to platforms for ocean thermal energy conversion (OTEC) applications.
OTEC systems are energy production systems that exploit the ocean temperature difference between the warm surface water and the cold water in deeper ocean strata. Typically an OTEC system includes a barge mounted plant and a large diameter cold water pipe. Cold water from the ocean depths is pumped to the surface through the cold water pipe. The cold water is then directed into an power module. The power module also receives warm water from the surface. The temperature differential between the cold water and warm water is then exploited in the generation of electric energy through well known OTEC techniques. Typically, cold water of about 4 degrees C. must be obtained. Obtaining such cold usually requires operating the OTEC system in water depths of at least 3,000 feet.
Generally, barge mounted OTEC systems are shut down when ocean wave conditions become rough. It is desirable to design an OTEC production platform that is stable enough to remain operational and permanently stationed even in rough seas (as used herein, stable generally refers to small motion response to waves, in particular small heave). However, as OTEC systems increase in size, platform stability becomes an increasing problem. OTEC systems are now being designed having a generating capability of 400 MW of energy or more. Such systems may require a cold water pipe of 100 feet in diameter that extends 3,000 feet below the ocean surface. Because of the massive size of the cold water pipe, wave induced heave, roll and pitch of the platform must be minimized since any large movement of the platform could damage the cold water pipe. Thus, it would be further desirable to design a high stability OTEC platform to accommodate large OTEC systems.
Semi-submersible platforms are often used for off-shore oil and gas drilling because of their great stability. A semi-submersible platform typically has a deck supported by columns and pontoons. The pontoons are submerged below the ocean surface and the columns extend upward from the pontoons to the deck. The pontoons and the columns provide both buoyancy and stability to the platform. Wave induced heave, roll and pitch are well known motion problems for drilling platforms. Platform stability is achieved by designing the shape and size of the columns and pontoons so that the wave forces on the platform tend to be balanced regardless of the wave direction and wave length. Generally, semi-submersible platforms are designed to balance the wave forces on the surface penetrating columns and the underwater body. Examples of semi-submersible drilling platforms that have addressed these problems are disclosed in U.S. Pat. Nos. 4,112,864 and 4,829,928 both to Bergman, and both expressly incorporated herein by reference. Typical platforms as disclosed in these patents have columns that have drafts of up to 150 feet deep.
The semi-submersible platforms designed for drilling purposes are unsatisfactory for OTEC applications for a number of reasons. First, such designs do not include a massive cold water pipe. Further, the stability of such designs is not high enough to prevent damage of the cold water pipe in extreme sea conditions. Moreover, the deep seas in which an OTEC system must operate expose the OTEC platform to higher wave forces than typically seen in oil and gas applications, thus requiring OTEC platforms to be more stable. Further, it is desirable for OTEC platforms to remain permanently stationed in all sea conditions. In addition, such oil and gas designs generally are not directed towards platforms that have structural components that extend as deep as an OTEC platform. Finally, such designs do not account for the other components of an OTEC system. For example, an OTEC platform may have fixed power cables that transmit power from the platform to shore. Additional platform stability is required to prevent undesirable movement of such power cables. Thus, it would be desirable to design a highly stable semi-submersible OTEC platform.